Static discharge for electro-explosive devices

ABSTRACT

A static discharge device for an electro-explosive having a pair of electrical lead wires with a bridge element connected there across and in contact with an explosive charge. The bridge element has saw tooth edges positioned away from the explosive charge to provide multiple selective static breakdown points.

[451 Aug. 21, 1973 ilnited States Patent Menichelli 8 2 U m W mm M mm W Pu M M Rm 3 m. C M

9 m mm m 6 mm M 5 Vincent J. Menichelli, Panorama City, Calif.

[73] Assignee: The United States of America as Primary Examiner-Verlin R. Pendegrass Attorney-R. S. Sciascia, J. A. Cooke, M. G. Roshin represented y the Secretary of the and R. J. Erickson Navy, Washington, DC.

Sept. 19, 1968 [22] Filed:

Appl. No.: 760,934

A static discharge device for an electro-explosive having a pair of electrical lead wires with a bridge element [52] U.S. 102/28 R F42b 3/18 connected there across and in contact with an explosive charge. The bridge element has saw tooth edges [58] Field of 102/28 positioned away from the explosive charge to provide multiple selective static breakdown points.

[5 6] References Cited UNITED STATES PATENTS 3 Claims, 2 Drawing Figures 3,306,202 2/1967 Menichelli et 102/28 Patented Aug. 21, 1973 FIG M 5 M I Q a 3 2 M v a o 3 k I V 7 44;? x MP. I H M m 4 0 5 v 0 3 4 4 2 I Vincent J. Menichelli INVENTOR AGENT STATIC DISCHARGE FOR ELECTRO-EXPLOSIVE DEVICES BACKGROUND OF THE INVENTION This invention relates generally to electric initiators for explosives, and more particularly to a static discharge device for an electric initiator for explosives.

Electrical initiators for explosive devices inherently experience a problem of premature detonation due to static discharges of large potentials within the electroexplosive devices. Conventional methods of overcoming the problem of premature initiation of electrical initiators have not been entirely satisfactory. One such prior art solution which has found wide acceptance in the art involves the use of an insensitive high explosive charge with an exploding bridge wire as the initiator. The bridge wires carries relatively high currents which are inadvertently applied to the device but when a sharp high voltage pulse of electrical energy is applied, the wire explodes and ignites the charge. Often it is found that inadvertently applied low voltages have heated the bridge wire to its fuzing temperature without initiating the explosive, but still rendering the device inoperative thereafter. Another device known to the prior art utilizes a conventional explosive and a bridge element which acts as a heat sink capable of initiating the charge upon reaching a preselected temperature. To prevent inadvertent initiation of such electroexplosive devices due to static discharges, one lead wire of the electro-explosive bridge circuit is grounded.

This, however, increases the probability of initiation SUMMARY OF THE INVENTION Accordingly, one object of this invention is to provide a new and improved static discharge device for ele ctro responsive devices.

Another object of the invention is the provision of a new and improved static discharge device which provides selective multiple discharge points to positively prevent inadvertent initiation of an electro-explosive device.

A further object of the instant invention is to provide a new and improved electro-exposive device which will reliably detonate upon demand and which is portected from inadvertent initiation due to static discharges.

Briefly, in accordance with one embodiment of this invention, these and other objects are attained by providing an electro-explosive device having a casing with an explosive charge therein, a pair of electrical lead wires providing electrical communication with the interior of the casing, a bridge element connected across the lead wires and in heat transfer communication with the explosive charge, wherein the bridge element has formed integrally thereon multiple static discharge points arranged adjacent to and about the casing.

BRIEF DESCRIPTION OF THE DRAWING A more complete appreciation of the invention and many of the attendant advantages thereof will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a longitudinal cross-sectional view through the electro-explosive device of the present invention; and

FIG. 2 is a sectional view along line 2-2 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing wherein like reference characters designate identical or corresponding parts throughout the several views, and more particularly to FIG. 1 thereof wherein the electro-explosive initiator according to the invention is shown as being formed of a cylindrically shaped housing or casing 10 of conventional type having a closed end 12 and an open end 14 with a main charge 16 contained within the lower portion thereof.

The electrically actuated means for initiating the explosive main charge 16 operates in the conventional manner by heating a heat sensitive explosive material by means of an electrical resistance element. The structure of the heating system is different from prior art structures, however, in ways which will be obvious from the following description.

A heat sensitive initiator charge 18 of conventional material such for example, as lead azide, is supported in contiguous relationship to the main charge 16 in a cup-shaped charge holder 20 which fits within casing 10. Charge holder 20 is provided with a central bore 22 and an upperwardly extending integral cylindrical wall 24. A flat insulating disc 26 is positioned in charge holder 20 to ensure electrical insulation between charge holder 20 and the electrical parts of the device. The resistance heating element or bridge element is disposed atop disc 26. As shown in FIG. 2, resistance element 28 is a flat serpentine shaped strip of suitable metal. In the preferred embodiment, bridge element 28 is of Evanohm having a bridge resistance of approximately one ohm, although it will be understood that other suitable materials may be employed as desired. In order to supply electrical energy to bridge element 28, a pair of lead-in wires or conductors 30 and 32 are provided which are connected across bridge element 28 at their lower ends. In this manner, heat is transmitted from resistance heating element 28 to the lead-in conductors 30 and 32 thereby providing for the lead-in conductors themselves to act as heat sinks. Completing the assembly is a cylindrical body 34 of glass or other suitable material having a pair of longitudinal bores 35 therethrough serving to support the conductors 30 and 32. A metallic sleeve 36 surrounds the top portion of body 34, permitting the parts to be joined together and sealed by a solder joint 38. Body 34 may also be of a material capable of conducting heat away from bridge 28, but this is not essential to the invention.

The heat sink effect of conductors 30 and 32 pro vides the desired safety in the device at low values of input current. At such low values, the resistance element 28 will heat up somewhat, but the heat will be conducted away through the lead-in conductors so that the heat sensitive charge 18 is not heated to its detonation point. By making the resistance element 28 flat, the relation between the resistance element and the lead-in conductors and that between the resistance element and the explosive is variable as hereinafter described Upon application of a higher value of current a or wattage, the relationship of the lead-in conductors and the bridge element is such that the lead-in conductors can not carry away sufficient heat to prevent actuation of the initiator charge. Under these conditions of course, detonation of the device occurs.

As hereinbefore stated, the use of a flat bridge element as the heating element makes it possible to vary the relationship between the heat lost from the strip to the heat sink and the heat lost to the explosive. The amount of heat lost to the heat sink is a function of the cross sectional area of the strip since heat cannot leave the central portion of the strip except by conduction through the area of the strip which is effectively in contact with the heat sink and the effective contact area is the cross sectional area. The loss to the explosive is a function of the amount of surface area of the strip in contact with the explosive. Thus, for a giving crosssectional area and length of strip, the loss to the heat sink is constant, and the loss to the explosive can be selectively controlled by varying the shape of the crosssection, by varying the thickness and the width of the strip, by placing more or less area in contact with the explosive. Construction of the bridge element 28 in a serpentine configuration permits the attainment of a high electrical resistance and large surface area. It should be noted that the rate of heating strip 28 is a function of the cross-sectional area. It therefore follows that the rate of heating of the strip is fixed for a given cross-sectional area and the relative rate of heating of the explosive can be varied by varying the shape of the strip. By the simple expedient of using a serpentine shaped ribbon or strip instead of the conventional cylindrical wire, a high degree of control over the current sensitivity and response time of the device is achieved.

As can more readily be seen in FIG. 2, discharge of a static potential which may build up on lead 30 or 32 is prevented from initiating charge 18 by constructing the bridge element 28 with a pair of sections 40 and 42 each in the shape of a chordal segment of a circle and each having a plurality of protuberances formed on the periphery thereof providing a multiplicity of discharge points 44 and 46 respectively and in close proximity to metal sleeve 36. In this manner any static potential building upon on leads 30 and 32 will be discharged through the multiple static discharge protuberances 44 and 46 onto metal sleeve 36, thus preventing a discharge across leads 30 and 32.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. An electroresponsive bridge element for an explosive device comprising a pair of flat electrical resistance sections each in the shape of a chordal segment of a circle bounded by a straight edge and a curved edge,

a plurality of static discharge protuberences extending from said curved edges, and

a serpentine connecting element extending between said straight edges.

2. An electroresponsive bridge element for an explosive device comprising a pair of flat resistance sections each in the shape of a chordal segment of a circle bounded by a straight edge and a curved edge,

a serpentine connection element extending between said straight edges, and

means formed continuously along said curved edges for concentrating static electric charges.

3. An electro-explosive device comprising:

a metallic casing;

an explosive charge disposed within said casing;

a pair of electrical lead wires extending within said casing;

means for providing electrical insulation between said lead wires and said casing;

an electroresponsive bridge element comprising:

A pair of sections, each in the shape of a chordal segment of a circle, electrically connected with one of said lead wires, and having a plurality of static discharge protuberances about the periphery thereof adjacent said casing and electrically insulated from said explosive charge; and

a serpentine element extending between and connected to said sections and in heat transfer relationship with said charge.

* 1 l i t 

1. An electroresponsive bridge element for an explosive device comprising a pair of flat electrical resistance sections each in the shape of a chordal segment of a circle bounded by a straight edge and a curved edge, a plurality of static discharge protuberences extending from said curved edges, and a serpentine connecting element extending between said straight edges.
 2. An electroresponsive bridge element for an explosive device comprising a pair of flat resistance sections each in the shape of a chordal segment of a circle bounded by a straight edge and a curved edge, a serpentine connection element extending between said straight edges, and means formed continuously along said curved edges for concentrating static electric charges.
 3. An electro-explosive device comprising: a metallic casing; an explosive charge disposed within said casing; a pair of electrical lead wires extending within said casing; means for providing electrical insulation between said lead wires and said casing; an electroresponsive bridge element comprising: A pair of sections, each in the shape of a chordal segment of a circle, electrically connected with one of said lead wires, and having a plurality of static discharge protuberances about the periphery thereof adjacent said casing and electrically insulated from said explosive charge; and a serpentine element extending between and connected to said sections and in heat transfer relationship with said charge. 